Algorithms for adjusting pre-1994 NADP/NTN data for the
effects of contamination from the lid o-ring. The "cor"
and "unc" subscripts designate the concentrations that
are corrected or uncorrected for the o-ring. Volume (Vol)
is in mL and pH is in pH units. Volume restriction
categories for the algorithms with volume and pH factors
are listed. These corrections effectively limit the size
of the corrections at very low and very high pH values

Changes in the intercept of the deseasonalized trend
model during 1995 relative to the 1983-1994 trend period.
Significance values, p, are those for a binary, 1983-94
vs. 1995, indicator variable added to the trend model

Title IV of the Clean Air Act Amendments of 1990 (CAAA-90) (Public
Law 101-549) seeks to reduce acidic deposition in the United States
through phased reductions in sulfur dioxide and nitrogen oxide
emissions. One of the first steps in assessing the effectiveness
of emissions reductions is to evaluate spatial and temporal trends
in sulfate (SO42- ), nitrate (NO3-), and hydrogen (H+) ion
concentrations in precipitation. Lynch et al. (1995a) reported the
most recent comprehensive summary of temporal trends in
precipitation chemistry in the United States using data from 58
National Atmospheric Deposition Program/National Trends Network
(NADP/NTN) sites from 1980 through 1992. Results showed widespread
declines in SO42- concentrations accompanied by significant
decreases in all of the base cations, most noticeably calcium
(Ca2+) and magnesium (Mg2+). As a result, only 17 of the 42 sites
with significant (p<0.05) decreasing SO42- trends had concurrent
significant decreasing H+ trends. Lynch et al. (1995a) cautioned
that they found considerable inter- and intra-regional variability
in pH trends even among multiple sites within the same state. They
speculated that significant site-specific changes in NO3- and
ammonium (NH4+) concentrations, as well as varying magnitudes of
SO42- and Ca2+ changes could help explain much of this variability.

In the National Acid Precipitation Assessment Program (NAPAP)
report on deposition monitoring, Sisterson et al. (1990) applied a
Kendall seasonal tau test for trend detection in the presence of
seasonal cycles (see Hirsch et al., 1982). Seasonal observations
were defined as monthly mean precipitation-weighted concentrations
or monthly depositions for each site. Data from six North American
networks, including NADP/NTN, were used in the analysis. Sen's
median slopes (see Hirsch et al., 1982) were calculated to estimate
the magnitude of changes in concentrations and depositions for
sites meeting predetermined data completeness criteria (Olsen et
al., 1990; Sisterson et al., 1990). The NAPAP trends analysis
addressed two time periods: a 39-site (24 NADP/NTN sites), 9-year
data set (1979-1987) and a 148-site (76 NADP/NTN sites), 6-year
data set (1982-1987). Statistically significant (p<0.05) trends
were found at a higher proportion of sites for all ions, except
NO3-, in the 9-year data set than during the 6-year period,
suggesting that the largest changes occurred early in the data
record from 1979-1982. Except for NO3- and potassium (K+), the
absolute magnitudes of the median percent changes were larger for
the 9-year period, as well. For the ions that most affected pH
(SO42- , NO3-, NH4+, and Ca2+), the strongest evidence for change
was in the base cation, Ca2+. Calcium decreased at 38 of the 39
sites over the 1979-1987 period; at 13 sites these changes, all
down, were significant (p<0.05). Sulfate decreased at 35 of the 39
sites; at seven sites these changes, all down, were significant
(p<0.05). Nitrate, NH4+, and H+ ions exhibited a more even split
of increasing and decreasing changes (Sisterson et al., 1990, pages
6-154 through 6-163). These results led the authors to conclude
that the acidity of precipitation did not decrease because of the
concurrent decrease in cation concentrations.

Lynch et al. (1995b) also evaluated trends in precipitation
chemistry at NADP/NTN sites for multiple summary periods. This
analysis utilized a general, linear least-squares model to evaluate
trends for three summary periods: 1980-1993, 1983-1993, and 1985-1993.
Major growth in the NADP/NTN occurred between 1980 and 1985
effectively tripling the number of network sites. Choosing
successively later start dates allowed comparison of results among
larger numbers of sites having greater spatial coverage. It also
allowed the assessment of the effect of summary period length on
trends. Regardless of the summary period length, the vast majority
of NADP/NTN sites exhibited decreasing SO42- concentrations.
Consistent with the findings of Sisterson et al. (1990), this
analysis also indicated that SO42- concentrations decreased more
rapidly during the early 1980s and less rapidly thereafter.
Nitrate and NH4+ concentrations exhibited considerable variability
with only a few sites showing statistically significant trends,
some positive and some negative. The larger SO42- decreases in the
early 1980s were similar to sulfur dioxide emissions changes, which
decreased more rapidly between 1980 and 1983, then vacillated about
a nearly constant rate in many states (Lins, 1987).

Hedin et al. (1994) reported steep declines in base cations in
precipitation from Sweden, the Netherlands, and the United States.
In the United States, annual volume-weighted mean concentrations of
SO42- and base cations (defined as the sum of non-sea-salt Ca2+,
Mg2+, Na+, and K+) were calculated using data from 32 NADP/NTN
sites (1979 or 1980 to 1990), nine Multistate Atmospheric Power
Production Pollution Study (MAP3S) sites (1978-1988), and the
Hubbard Brook Experimental Forest (HBEF) (1965-1989). A regression
of these annual means against time in years yielded trend estimates
for SO42- and cations (NH4+, Ca2+, Mg2+, Na+, K+). At the HBEF and
at NADP/NTN sites in the Northeast, Southeast, and Midwest, both
SO42- and base cations decreased and the trends were significant
(p<0.05 or p<0.001). At the nine MAP3S sites, SO42- decreases were
also statistically significant; however, base cations decreased at
only five sites, none of which were statistically significant.
Hedin et al. (1994) concluded that recent declines in both base
cation and SO42- concentrations had offset one another in varying
proportions in many regions in the Northern Hemisphere.

Other authors have also focused their attention on describing
trends in precipitation concentrations or depositions (for eastern
North America, see Sirois, 1993 or Oehlert, 1993; for NADP/NTN
data, see Baier and Cohn, 1993; for MAP3S data, see Dana and
Easter, 1987; for data from the Netherlands, see Ruijgrok and
Romer, 1993 or Buishand et al., 1988; for data from Texas, see
Kessler et al., 1992; and for data from New York, see Hirsch and
Peters, 1988). All have shown general decreasing trends in SO42-
and base cation concentrations, with small and generally
statistically insignificant changes in free acidity. In these
studies, both parametric (e.g., Buishand et al., 1988) and
nonparametric (e.g., Baier and Cohn, 1993) approaches have been
used.

Measuring and reliably quantifying trends in precipitation
chemistry are essential tools for assessing the effectiveness of
sulfur and nitrogen emissions reductions programs designed to
protect the environment. Trend estimation techniques were compared
at a recent workshop (Holland et al., 1995). Four approaches were
examined, including the linear least-squares model used in this
analysis and several modifications of the seasonal Kendall test
(Hirsch et al., 1982). In one case, a modification of Sen's slope
estimator (Hirsch et al., 1982) was used to estimate the annual
percentage change in precipitation-weighted mean concentrations.
This comparison indicated that all four approaches were valid and
generally yielded similar long-term trend results, although
strengths and weaknesses were noted for each approach. The linear
least-squares approach provided the added advantage of quantifying
seasonal changes in concentration over time.

Phase I of Title IV of the CAAA-90 requires specific reductions in
sulfur dioxide emissions on or before 1 January 1995 at selected
electric utility plants, the majority of which are located in
states east of the Mississippi River. As a result of this
legislation, large reductions in sulfur dioxide emissions are
likely to have occurred in 1995 which should have affected SO42-
and H+ concentrations, and to a lesser extent NO3- concentrations,
in precipitation in this region. The purpose of this study was to
evaluate this effect, if any, at NADP/NTN sites in the region in
1995. This assessment was based on a comparison of observed 1995
SO42- , H+, and NO3- concentrations at these NADP/NTN sites with
estimates obtained from linear least-squares trend models of
precipitation chemistry data collected from 1983 through 1994. The
1983 through 1994 summary period was selected for the following
reasons: (1) the 1980-1994 record limits the number of sites to 57,
too few for meaningful inter- and intra-regional comparisons; (2)
the 1985-1994 summary period increases the number of sites to 168,
but reduces the strength of the trend models estimates because of
the shortness (10 years) of record; (3) the 1983-1994 summary
period represents the best compromise between both the spatial and
temporal strengths of the analysis; (4) the 1983-1994 summary
period avoids potential start-up problems associated with field
sampling and laboratory protocols; (5) the 1983-1994 period has
stable emissions relative to the early 1980s (Lins, 1987); and (6)
the National Trends Network (NTN) became a formal part of NADP in
1983, so many of the sites added in the 1983-1994 summary period
over the 1980-1994 period were NTN sites.

Since its inception in 1978, NADP/NTN sampling protocols required
site operators to send precipitation samples to the Central
Analytical Lab (CAL) at the Illinois State Water Survey in the
~14-liter HDPE (high density polyethylene) buckets used for collection.
The bucket lid contained a rubber o-ring, which sealed the lid and
bucket, preventing leaks during shipment. This o-ring was found to
be a source of many of the same cations and anions measured in
precipitation. For most samples, the concentration biases due to
o-ring contamination were small and unimportant. Most affected
were free H+ concentrations (pH) at sites in the western states,
where pH values are typically above 4.8. Samples lost free acidity
due to the o-ring so sample pHs were biased high. Efforts to
improve lid cleaning procedures reduced, but could not eliminate,
o-ring contamination.

Although o-ring contamination was persistent, occasional changes by
the lid manufacturer added to the uncertainty in the size of the
bias over time. This had the potential for interfering with long-term
trend analyses. Studies had shown that samples sent to the
CAL in bottles had much smaller losses of acidity than samples sent
in the buckets with the lid o-ring. As a result, the NADP/NTN
elected to change its procedures effective 11 January 1994, ending
the use of the buckets for sample shipment and eliminating any
contact of the sample with the o-ring. After 11 January 1994, a
snap-on lid with no o-ring was used to cover the collection bucket
during transport from the site to the field laboratory and a one-liter,
wide-mouthed, HDPE bottle was used for shipment to the CAL.

A special study was conducted at 11 sites scattered across the
network to assess the size and direction of the concentration
change of each analyte due to lid o-ring contamination. The
results of this study were analyzed with the intent of identifying
a set of algorithms that could be used to adjust the pre-1994 data
for the effects of o-ring contamination. Side-by-side samples were
collected at these sites, one using the old procedure with the
bucket and lid with the o-ring and the other using the new
procedure with the bottle. Paired concentration differences were
calculated from the two samples at each site. Nonlinear, least
squares regressions were performed to evaluate these differences
for dependence on sample volume or on pH or on a combination of
these factors. CAL experimental results suggested that both
factors could be important in explaining the differences between
the bucket and bottle samples. Specifically, these results
indicated that as the volume increases the o-ring contamination
increases, approaching a maximum value. For a fixed volume, the o-ring
contamination is large at low and high pH values and small at
mid-level pHs. These volume and pH relationships dictated the form
of the equations used in the analysis.

Six different models (algorithms) per analyte were evaluated for
each of the conservative ions. These included: (1) a constant
mass model, (2) a simple linear regression model, (3) a mass
difference model as a function of sample volume, (4) a mass
difference model as a function of sample volume and pH, (5) a power
law model, and (6) hybrids of 2 and 3 or 2 and 4. To select the
best algorithms, the root mean square (RMS) bucket/bottle
concentration differences and the RMS corrected bucket/bottle
concentration differences from the six models were calculated. A
sensitivity test of the RMS calculations was performed using a
"bootstrap" experiment with 100 repetitions. Following these
experiments, the most robust algorithms with the lowest RMS of the
corrected bucket/bottle concentration differences were identified.
Using the overall "best fit" models for the conservative ions, the
pH (H+) correction was evaluated. This evaluation revealed that
the best model was one based on adding the effects of all of the
corrections plus an unmeasured cation (probably zinc, which can be
leached from the lid seal), then calculating a corrected pH. The
equations that best fit the special study results for all
measurements are presented in the Appendix (Table A.1). Four of
the correction equations (K+, NO3-, Na+, and SO42- ) incorporate
both sample volume and sample pH as predictor variables. For four
others (Ca2+, Mg2+, NH4+, and Cl-), sample volume alone was the
best predictor.

A comparison of volume-weighted mean annual concentrations in
precipitation for corrected and uncorrected NADP/NTN data from
1983-1993 (Appendix, Table A.2) illustrates the effects the
corrections (Appendix, Table A.1) had on NADP/NTN precipitation
chemistry data. At all sites, the corrections resulted in lower
mean annual concentrations for all ions, except H+. Hydrogen ion
concentrations increased as reflected in the lower pH values. With
a few exceptions, the reductions in pH were less than 0.04 unit in
the eastern states; reductions in pH were higher in the West, with
some sites experiencing reductions in mean annual pH of more than
0.3 unit. Sulfate concentrations generally decreased from 0.03
mg/L to 0.04 mg/L in the western region of the country; at Eastern
sites, SO42- concentration decreases were about 0.01 mg/L with
concentration changes at some sites exceeding 0.1 mg/L. Decreases
in NO3- concentrations were fairly uniform across the country and
averaged around 0.03 mg/L. Chloride concentrations decreased
approximately 0.005 mg/L and did not exhibit significant regional
differences. Ammonium concentrations decreased approximately 0.01
mg/L and were considerably more variable in the western than the
eastern states. Calcium and Mg2+ concentrations decreased between
0.003 and 0.004 mg/L at most sites and exhibited very little
spatial variation across the country. A similar pattern and
magnitude of change was also evident for K+ and Na+ concentrations.
Overall, the correction equations did not result in any major
aberrations in the data set and are consistent with results from
bucket versus bottle comparison studies.

Temporal trend analyses covered the entire NADP/NTN network,
included all analytes except orthophosphate, and was based on
corrected data through 1993 and 1994 data. Trends in analyte
concentrations were examined for the 1983-1994 summary period.
Results of this analysis provided a baseline against which 1995
concentrations were contrasted to assess the effects of the CAAA-90,
Phase I emissions reductions on precipitation chemistry.

In the trend analysis of the summary period (1983-1994), weekly
precipitation volume and corrected chemical observations were
accumulated into bi-monthly precipitation totals and volume-weighted
mean concentrations of H+ (from pH), SO42- , NO3-, Cl-,
NH4+, Ca2+, Mg2+, K+, and Na+ ions. Orthophosphate was omitted
because a large percentage (>80%) of weekly samples have
concentrations below the analytical detection limit (0.003 mg/L).
Only valid weekly samples with a complete set of analyses were used
to calculate bi-monthly volume-weighted mean concentrations. Sites
and bi-monthly records were selected for the trend analysis
according to the following completeness criteria:

Only those monitoring sites having weekly precipitation
chemistry records from January 1983 through December 1994, were
considered. At least 75 percent of the precipitation recorded
during this summary period had to have valid chemical analyses in
order for the site's data to be accepted for trend analyses.

For a bi-monthly record to be accepted, a valid analysis for
each ion had to be available for at least 75 percent of the bi-monthly
precipitation.

During each bi-monthly period, at least 50 percent of the
weekly samples having sufficient volume for analysis (>35
mL) had to have a valid analysis for each ion.

Trends in ionic concentrations in precipitation at each site were
evaluated using a two-stage, least-squares general linear model
(SAS Inst, 1988). This model was developed by the principal
investigators for detecting and quantifying trends in precipitation
chemistry data that exhibit strong seasonal patterns (Lynch, et
al., 1995a). The form of the model for both
stages was

where,

Cy = estimated concentration of a given ion at time y.

b0 = intercept.

by = slope of the long-term log-concentration trend.

y = mid-point of the bi-monthly observation period
expressed as decimal years. For example, y for a
May-June 1990 observation was coded as 90+(5/12)
or 90.4167.

bs = adjustment to estimate for bimonthly period, s.
The array of 6 bs coefficients account for the
seasonal variation in precipitation chemistry.

Is = an element of an array of 6 indicator variables
set to 1 for bi-monthly period, s, and set to 0,
otherwise.

Log-transformed concentrations were used because the model
residuals have a more nearly normal distribution (Lynch et al.,
1995a). After initially fitting the model to a site's
concentration data (expressed as µeq/L) for a given ion,
studentized residuals were calculated. Bi-monthly observations
having a studentized residual >3.5 in absolute value were
eliminated from the data set and a second calculation of model
coefficients was performed using the remaining observations. The
selected cut-off value applied to the studentized residuals would
be exceeded by chance at a rate less than 0.001 under the
assumption of normally distributed residuals of constant variance.

A tabular summary of the trend results is presented in Table A.3
(Appendix). This summary table contains information on the
direction (byear) and statistical significance (p) of the trend in
each analyte for each site that met the above completeness
criteria. Sites are identified by their NADP/NTN CAL code. The
Mississippi River was used to segregate those sites located in the
eastern and western sections of the country. The number of sites
exhibiting increasing trends and the number exhibiting decreasing
trends are presented in Table 1. These sites are further
subdivided by statistical significance (p<0.05).

The direction and statistical significance (p<0.05) of trends for
each analyte are also presented graphically in Figures 1-9. Upward
or downward pointing triangles indicate the direction of the trend.
A solid triangle indicates a statistically significant (p<0.05)
trend; an open triangle indicates that the trend is not significant
(p>0.05). NADP/NTN sites located in Alaska and Hawaii are not
shown on the maps but were included in the analyses if they met the
completeness criteria.

Changes in ionic composition of precipitation at each site from the
beginning (1983) to the end (1994) of the summary period were
calculated as the difference between the average of six bi-monthly
mean concentrations (µeq/L) estimated from the models for 1983 and
1994. Cation and anion concentration (µeq/L) changes and percent
changes from 1983 to 1994 are presented in Table 4.A (Appendix).
Table 2 lists the mean concentration (µeq/L) changes and percent
changes for all sites and for only sites with statistically
significant (p<0.05) trends. The coincidence of significant
(p<0.05) decreasing SO42- and H+ concentrations is summarized in
Table 3. Sites are identified by the NADP/NTN CAL code.

Phase I of the CAAA-90, Title IV was implemented on 1 January 1995.
One-hundred and ten (110) electric utility plants were affected in
21 states, 17 of which are located east of the Mississippi River.
Sixty-three (63) of these targeted plants are located in states in
the Ohio River Valley. Notwithstanding the effects of trades of
emissions allowances, large reductions in sulfur dioxide emissions
are likely to have occurred in 1995, particularly in the eastern
states. These reductions should have affected precipitation
chemistry, particularly SO42- and H+ concentrations, and to a
lesser extent NO3- concentrations. In order to evaluate this
effect, if any, at NADP/NTN sites in the eastern states in 1995,
the linear least-squares models discussed above were used to
estimate 1995 bi-monthly and annual mean concentrations for each
analyte. The model estimates were compared to actual bi-monthly
volume-weighted mean concentrations for each NADP/NTN site that met
the above completeness criteria. Because the bi-monthly means were
summed to obtain annual means, only 109 sites with six valid 1995
bi-monthly means were included in this analysis. For comparative
purposes, separate, identical analyses were conducted for sites
located east and west of the Mississippi River. The impact of
Phase I emissions reductions in the western states should have been
minimal given that only 16 of the affected plants are located in
this region (Missouri-8, Iowa-6, Kansas-1, Minnesota-1). As a
result, a comparison of predicted with observed concentration means
in both regions provides a means of evaluating model performance
and the effect of CAAA-90 emissions reductions on precipitation
chemistry.

The results of the comparisons of 1995 measured and estimated
bi-monthly and annual mean concentrations for individual sites
located in eastern and western regions of the country are presented
in Tables 4, 5, and 6 for SO42- , H+, and NO3- concentrations,
respectively. Similar comparisons for the remaining cations and
anions are presented in the Appendix (Tables A.5-A.10). Eastern
results are further stratified into Northeast (NE) and Southeast
(SE) regions. Table 7 list the frequency of occurrence of observed
1995 cation and anion concentrations that were less than predicted
concentrations. Regional mean and percent departures from the trend
model estimates are presented in Table 8 for annual and bi-monthly
cation and anion concentrations.

One way to assess the statistical significance of the effects of
Phase I emissions reductions on precipitation chemistry is to
determine the significance of the deviations of the 1995 bi-monthly
observations from the trend model estimates derived from the
1983-1994 data. Because only one year (1995) of data is available
subsequent to Phase I implementation, quantifying a slope change in
the long-term trend is inappropriate. However, the data do permit
estimation of changes in the intercept of the trend line coinciding
with the implementation of emission reductions. Estimating the
change in the intercept is appropriate because it directly
quantifies the "step-function" that would occur in precipitation
chemistry if emissions were reduced suddenly, i.e., large reductions
over a short period of time. The change in intercept for the 1995
data was estimated by adding a binary indicator variable to the
seasonal trend models. This indicator variable was given a value of
1 for 1995 observations and a value of 0, otherwise. This
variable's estimated coefficients and its significance are presented
in Table A.11 (Appendix) which indicates in what direction and at
which sites the 1995 data departed from the historical trend.
Because there are only six bi-monthly observations in 1995, the
power of this statistical test for change is rather low.
Nevertheless, the results of this analysis reaffirm the results from
the linear least-squares models and illustrate that the 1995
precipitation chemistry data were different from the historical
trends at NADP/NTN sites located in the Eastern United States.
Sulfate and H+ concentrations in 1995 at 12 of the 62 sites located
east of the Mississippi River were significantly (p<0.05) different
when compared to the historical trend models at these sites; for
NO3- concentrations, only two sites were significantly different,
both positive indicating higher concentrations in 1995 than the
model estimates.
In addition to the above analyses, 1983-1994 trends for selected
sites are presented graphically by plotting observed bi-monthly mean
concentrations along with two corresponding estimates of
concentration against time. One set of estimates was from the
least-squares general linear models described above, the other set
was from a LOWESS regression of the observed bi-monthly mean
concentrations. The LOWESS smoothing method, described by Cleveland
(1979 and 1985), was added because it does not assume a functional
relationship between concentration and time and can depict
nonlinearities in trends. The LOWESS method was not used to
statistically assess concentration trends because assessment of
changes in ionic concentration from one time to another, by
definition, involves linear hypotheses and because LOWESS
regressions do not provide an overall test of trend or model fit for
the data set as a whole. The "moving window" of data points was set
at 2 years (12 bi-monthly points) before and after the date to be
estimated. The distance weighting function for the LOWESS
regressions was,

where,

pi = |xi-xt|/W for |xi-xt| < W; otherwise, 1

xt = date of point to be estimated in decimal years

xi = date of ith sample point in decimal years

W = width of moving window in each direction (i.e.,
2.0 years)

The robustness weights for the second stage of the LOWESS
estimation procedure were calculated as,

where,

pi = (|ri|/R) for |ri| < R; otherwise, 1.0

ri = studentized residual of ith sample point from
the first stage LOWESS regression

R = Maximum absolute value of studentized residual
for sample points to be used in the second
stage regression. R was set to 4.

For comparison purposes, a disjunct LOWESS regression line was
plotted for the six bi-monthly mean concentrations in 1995. This
LOWESS regression was based on 1993-1995 bi-monthly mean
observations. It was separated physically from the 1983 through
1994 LOWESS line to emphasize the step-function change in 1995 from
the preceding 12-year summary. Six examples of H+, SO42- , and NO3-
concentration trends showing the linear model (solid line) and
LOWESS regression (dashed line) and the observed bi-monthly mean
concentrations (solid circles) are presented in Figures 10-15 for
the 1983-94 and 1995 summary periods. Figures 10 and 11 (KY03 and
IL63, respectively) illustrate a dramatic change in 1995 bi-monthly
mean concentrations relative to the historical trend, while Figures
12 (NC36) and 13 (VT01) illustrate a moderate decrease and Figures
14 (IN34) and 15 (MA08) illustrate no change. The remaining cation
and anion comparisons for these six sites appear in the Appendix
(Figures A.1-A.12).

Color-scaled raster maps based on surface estimation algorithms are
frequently used to display precipitation chemistry data over a
region. This approach was used to illustrate the departures of 1995
observed concentrations (µeq/L and percent) from the trend model
estimates for 1995. Maps were prepared for the eastern half of the
country, where the greatest impact of Phase I emissions reductions
was most likely to occur. The differences were plotted using the
Multiquadic Equations (MQE) surfacing function described by Hardy
(1971) and evaluated as a tool for depicting regional wet deposition
by Grimm and Lynch (1991). Also included is a color-scaled raster
map showing deviations in 1995 precipitation volumes from the 1983
through 1994 annual average volumes. All four maps are based on
data from the 109 sites included in the analysis of the
effectiveness of the CAAA-90 discussed above. Results are presented
in Figures 16, 18, 20, and 22 for SO42- , H+, NO3-, and precipitation
volume, respectively; percent differences are presented in Figures
17, 19, 21, and 23, respectively. The location of NADP/NTN sites
are indicated by a plus (+) sign.

Sulfate concentrations at 92% of the NADP/NTN monitoring sites in the
United States have decreased since 1983; the trends are statistically
significant (p<0.05) at 38% of the sites (Table 1, Figure 1). No
major regional (east vs west) differences in the number and percentage
of sites exhibiting decreasing SO42- trends are
evident. Sites with increasing SO42- trends
(Table 1) are also uniformly distributed across the country; however,
only one of the increasing SO42- concentration
trends is significant (p<0.05). The mean change in
SO42- concentrations across the United States
from 1983 to 1994 was -4.80 (16.4%) compared to -7.69
µeq/L (25.9%) at
sites with statistically significant trends (Table 2). Sulfate
concentrations have decreased more rapidly in the eastern states (5.94
µeq/L vs 3.71 µeq/L), although the mean percentage change has
been greater in the western states (18.9% vs 13.8%). This pattern is
also evident at sites with significant (p<0.05) trends; however, the
concentration and percentage decreases are much larger (Table 2).

Nitrate concentration trends do not exhibit a consistent spatial
pattern (Figure 2). The number of sites exhibiting increasing
trends is nearly equal to the number exhibiting decreasing trends
(Table 1). Perhaps of greater importance, a larger percentage of
sites with increasing trends (14%) are significant (p<0.05), while
only two of the 153 sites included in this analysis have significant
decreasing trends. A larger percentage of sites with increasing
trends are located in the western states. As expected from these
results, the network mean change in NO3- concentrations was very
small (<0.5 ) and positive (Table 2). However, the mean NO3-
concentration at the 23 sites with statistically significant trends
(2 decreasing, 21 increasing) increased 51.6% (3.34 µeq/L). The
largest increases in NO3- concentrations occurred in the western
states.

Ammonium concentrations increases were larger and more widespread
than NO3- increases (Table 2, Figure 3). Eighty percent (80%) of
the sites in the NADP/NTN exhibited increasing NH4+ concentrations
since 1983 (Table 1); at 22% of the sites the trends are significant
(p<0.05). Only one of the sites has a statistically significant
decreasing trend. Although regional patterns are similar, the
largest number of sites with significantly increasing trends are
located in the western states. The average increase in NH4+
concentrations at these sites is 6.40 µeq/L, an increase of 86.6%
over 1983 levels. The network-wide increase in NH4+ concentrations
since 1983 is 2.33 µeq/L (28.2%). Clearly, nitrogen concentrations
(both NO3- and NH4+) in precipitation have increased in the United
States since 1983.

Calcium (Figure 4), Mg2+ (Figure 5), and K+ (Figure 6)
concentrations decreased markedly in the United States since 1983.
Very few sites exhibited increasing concentrations (Table 1): Ca2+
(9 sites), Mg2+ (2 sites), and K+ (21 sites); only two of these
trends (both K+) are significant (p<0.05). Decreasing trends are
significant at 52% of the sites for Ca2+, 77% for Mg2+, and 38% for K+. A higher percentage of these sites is located in the eastern states. Mean percentage decreases in these cations are fairly
consistent across the United States, 23.5% for K+, 27.9% for Ca2+
and 39.7% for Mg2+ (Table 2). Despite regional similarities in
percent decreases, the concentration (µeq/L) changes have been
consistently larger in the western portion of the country for all
three cations. These same patterns are evident when only sites with
significant trends are compared, although the percent and µeq/L
changes are larger. Clearly, base cation concentrations have
decreased over the past 12 years. Similar results have been
reported by Lynch et al., (1995a, 1995b); Hedin et al., (1994); and
Sisterson et al., (1990).

Sodium and Cl- concentrations generally decreased across the country
(Figures 7 and 8); trends are significant (p<0.05) at 20% of the
sites for Na+ and 37% for Cl- (Table 1). Regional differences in
both Na+ and Cl- trends are evident, with slightly more significant
Cl- and Na+ trends in the western than eastern region of the
country. Across the United States, Na+ and Cl- concentrations
decreased approximately 20% on average since 1983 (Table 2). At
sites with significant trends, Na+ and Cl- concentrations are 36.8%
to 32.3% lower. On a concentration basis, both Na+ and Cl-
decreased 1.5 µeq/L since 1983.

Like SO42- concentrations, the majority of sites (81%) exhibited
decreases in H+ concentrations from 1983 through 1994 (Table 1,
Figure 9). However, only 39 sites (25%) have statistically
significant H+ decreases, 20 in the western states and 19 in the
eastern states. Of the 29 sites exhibiting H+ increases, only one
is significant. Free acidity in precipitation across the United
States decreased by 2.68 µeq/L (13.4%) on average since 1983, with
the largest concentration (smallest percentage) changes occurring in
the East (Table 2). This same pattern is evident when only sites
with significant trends are compared, although both the percentage
and concentration decreases are more than twice as large. The lack
of consistency between sites with both significant decreasing SO42-
and H+ concentrations (Table 3) suggests that concurrent changes in
other ions have offset varying amounts of the SO42- decrease. As a
result, commensurate reductions in H+ did not occur. A similar
hypothesis has been suggested by Lynch et al., (1995a); Hedin et
al., (1994); and Sisterson et al., (1990). The ions most frequently
offsetting the SO42- reductions are the acid neutralizing cations,
Ca2+ and Mg2+. However, at some sites, increasing NO3-
concentrations have offset SO42- reductions leaving small changes in
free acidity (Lynch et al., 1995a).

Sulfate and H+ concentrations in the eastern states in 1995 were
considerably lower than predicted from the trend models for the
1983-1994 reference summary period (Tables 4 and 5); NO3-
concentrations remained relatively unchanged (Table 6). Two
examples each of relatively large decreases (KY03 and IL63),
moderately large decreases (NC36 and VT01), and little to no change
(IN34 and MA08) in 1995 bi-monthly mean SO42- and H+ concentrations,
compared to the 1983-1994 trend models, are shown in Figures 10-15,
respectively. Site selection was based on change in SO42-
concentrations in 1995 relative to the historical trend at each
site. A LOWESS regression line (dashed line) is included to
illustrate the step-function change in SO42- and H+ concentrations
that occurred in 1995. Corresponding base cation and Cl-
concentrations at these sites are shown in Figures A.1-A.12 in the
Appendix.

Mean annual H+ and SO42- concentrations in 1995 were below predicted
values at 88.7% and 79.0%, respectively, of the 109 sites that met
the data completeness criteria (Table 7). By comparison, only 31.9%
and 36.2%, respectively, of the Western sites were below modeled
estimates. On average, 1995 measured SO42- concentrations in the
eastern states were 4.01 µeq/L (9.8%) below modeled estimates (Table
8). The SO42- decreases were substantially larger than 4 µeq/L in
much of the Northeast. In contrast, at sites west of the
Mississippi River 1995 SO42- concentrations averaged 1.37 µeq/L
(7.5%) higher than the modeled estimates (Table 8). The spatial
pattern of H+ decreases in the East was virtually the same as the
pattern of SO4 decreases, although the magnitude of the
concentration and percentage decreases were even larger than SO42-
decreases (Table 8).

Unlike SO42- and H+ concentrations, NO3- concentrations in 1995 were
above predicted concentrations at the majority of sites located in
both regions of the country (Table 7). Approximately 61% of the
sites in the East recorded higher NO3- concentrations in 1995 than
predicted from the 1983-1994 models; 57% of the western sites were
above model predictions (Table 7). Nitrate concentrations in 1995
were 0.97 µeq/L (4.6%) and 0.76 µeq/L (3.8%) above modeled estimates
in the eastern and western states, respectively (Table 8). In
addition, there was no evidence of sharp drops in the Northeast as
there was for SO42- . These results suggest that nitrogen oxides
emissions were not significantly affected by Phase I of the CAAA-90,
Title IV in 1995, at least not on a broad regional basis.

Color-scaled raster maps of the changes in SO42- , H+, and NO3-
concentrations are shown in Figures 16, 18, and 20, respectively;
percent changes in these ions are presented in Figures 17, 19, and
21, respectively. The largest reductions in SO42- and H+
concentrations occurred along the Ohio River Valley and in states
located immediately downwind of this region. Although no emissions
data are presented in this analysis, this region was clearly
targeted for major reductions in sulfur dioxide emissions by Phase I
of the CAAA-90, Title IV. In fact, of the 110 plants affected by
Title IV, 63 are located in this region. Downwind of the Ohio River
Valley, e.g., New England or the southeastern states., decreases in
both SO42- and H+ concentrations in 1995 were smaller. There are
only six affected sources in New England and 18 in the Southeast
that were targeted for reductions in emissions in Phase I of the
CAAA-90, Title IV. Another important feature of these maps is that
the SO42- reductions in the eastern states are roughly matched in
magnitude and location by H+ reductions. The largest decreases
occur in the Ohio River Valley and the northern portion of the
Mid-Atlantic region.

To examine whether the sharp drop in SO42- and H+ concentrations in
1995 were due to precipitation anomalies, a comparison of bi-monthly
and annual precipitation volumes at each site with their 1983-1994
means was undertaken. Results of this analysis are listed for each
site in Table A.12 (Appendix). A summary of these results is
presented in Figures 21 and 22, which depict departures of 1995
annual precipitation from the mean annual volume during 1983 through
1994. A comparison of the precipitation volume color-scaled map
(Figure 21) with the SO42- (Figure 16) and H+ (Figure 18)
concentration maps reveals that where 1995 SO42- and H+
concentrations were higher than trend model estimates (e.g., the
southwestern portion of the Eastern United States and a region south
and east of Lake Michigan), 1995 volumes were below the 1983-94
averages. Ionic concentrations in weekly NADP/NTN samples depend on
precipitation volume (Baier and Cohn, 1993). Low precipitation
volumes are associated with high concentrations and vice versa.
When these factors are taken into consideration, the actual
reductions in SO42- and H+ concentrations across much of
Pennsylvania, Western New York, and the southern portion of New
England in 1995 would have been greater had a more "average" amount
of precipitation occurred in these regions.

A comparison of the differences in 1995 annual and bi-monthly mean
concentrations for those cations (NH4+, Ca2+, Mg2+, K+, and Na+) and
Cl- that would not have been substantially affected by Phase I
sulfur dioxide reductions (Tables A.5-A.10, Appendix), supports the
argument that the changes in SO42- and H+ concentrations in 1995
were not the result of lower precipitation volume. Precipitation
volumes would not selectively affect H+ and SO42- concentrations;
all ionic concentrations would be affected. A review of the
individual site data in Tables A.5-A.10 (Appendix) and observed
differences in NO3- concentrations (Figure 20) and percent changes
(Figure 21) support this statement. It is not apparent that NO3-
concentrations were affected over the eastern states by Phase I
emissions reductions, only that NO3- was affected by the below
average precipitation volumes. For example, regions with higher
than predicted 1995 NO3- concentrations had below average
precipitation and regions with lower concentrations in 1995
generally had above average precipitation. Although nitrogen
emissions may have been affected by the CAAA-90, Title IV, the
reductions would have been much smaller than the sulfur dioxide
reductions. In addition, since Phase I, Title IV of the CAAA-90
targeted only stationary (utilities) sources and since these sources
contribute only one-third of the total United States nitrogen oxide
emissions (Placet, 1990), it is highly unlikely that large regions
of the East would experience major reductions in nitrate
concentrations in 1995.

Clearly, implementation of Phase I of the CAAA-90, Title IV (Public
Law 101-549), has resulted in lower sulfate concentrations in
precipitation in the Eastern United States, particularly along the
Ohio River Valley and in the Mid-Atlantic region. Concurrent with
these sulfate reductions have been similar (nearly one for one)
reductions in hydrogen ion concentrations. In contrast, nitrate
concentrations, as well as chloride and base cations, were not
affected. Although emissions data were not included in this
analysis, maximum reductions in sulfate and hydrogen ion
concentrations occurred in the same area as and immediately downwind
of most of the major stationary sources targeted by Phase I of the
CAAA-90, Title IV. Precipitation deviations from the long-term
(1983-1994) average cannot explain the observed decreases in sulfate
and hydrogen ion concentrations in 1995. For one, precipitation
volumes would not selectively reduce only sulfate and hydrogen ions.
Other ions would be similarly affected, but they were not. For
another, lower precipitation volumes are associated with higher
concentrations, and most of the eastern states had below average
precipitation volumes in 1995. Indeed, the lower volumes resulted
in higher 1995 concentrations for virtually all ions except sulfate
and hydrogen. These two ions dropped independent of precipitation
volume. These results clearly support the conclusion that Phase I
of the CAAA-90, Title IV, has reduced acid deposition (acid rain) in
the Eastern United States.

Acknowledgment---This work was supported in part by the National
Atmospheric Deposition Program/National Trends Network (NADP/NTN)
through funds provided for its Central Analytical Laboratory at the
Illinois State Water Survey. The NADP/NTN is a NRSP-3 Project of
the State Agricultural Experiment Stations (SAES). Funding is
provided by federal and state agencies, universities, public
utilities and industries, as well as the SAES.